The introduction and expression of a normal protein by gene transfer techniques may result in the development of neutralizing immune responses as the normal protein may be recognized as non-self in those with inherited disorders of proteins. The success of gene therapy is dependent on the development of a state of immunological tolerance or anergy to protein products; achieving this could have an impact on the treatment of human disorders of secretory or intracellular proteins and neuromuscular disorders where inhibitory antibodies can develop; this remains an important and unresolved issue with present day protein-based therapy and future gene therapy trials. While mice have been the experimental model for most immunologists and the study of the murine immune response has yielded important insights, the failure of these models to predict the efficacy of gene therapy in humans is a major limitation. Accordingly, the current proposal will study the immune response to xenogenic transgene-encoded proteins expressed by AAV vectors in the nonhuman primate where the ontogeny on the immune system is similar to humans, and test the hypothesis that exposure to these persistent antigens, beginning either in utero or in the early neonatal period, will result in the induction of immunological tolerance. In addition, AAV expressing xenogenic factor VIII will be administered to the monkey fetus and neonate as a model for hemophilia A gene therapy to prevent inhibitor development to factor VIII, the treatment goal for this area of investigation. Preliminary data: Our research group has administered AAV expressing ovalbumin, an immunogenic and well- characterized protein, to newborn monkeys and have demonstrated a lack of immune response to this xenogenic protein while protective immunity to AAV has remained intact; augmented expression was possible with subsequent postnatal AAV administration without a humoral response to the xenogenic protein but did require AAV serotype switching. In Aim 1, AAV will be administered in utero and to neonatal nonhuman primates to study the mechanism of immune unresponsiveness to the foreign transgene-encoded protein. In Aim 2, we will assess the immune response to the viral vector administered in utero and the neonate. Finally, we will assess immune responses to the xenogenic factor VIII as a model for hemophilia A gene therapy and inhibitor formation prevention. These studies will determine if a clinically acceptable method to establish tolerance to immunogenic proteins is possible by initiating transgene expression while immunologic ontogeny may not be completely developed in a nonhuman primate model that could replicate a strategy for human intervention.